Process of aromatic hydrocarbon oxidation



Sept. 17, 1968 J BERTHQUX ET AL 3,402,184

PROCESS OF AROMATIC HYDROCARBON OXIDATION F iled Oct. '24, 1965 1e cl INVENT OR f Jean Ber/hoax Claude Gorbe/of-Brril/an' United States Patent 3,402,184 PROCESS OF AROMATIC HYDROCARBON OXIDATION Jean Berthoux and Claude Gerbelot-Barrillon, Lyon, France, as'signors to Progil S.A., Paris, France, a corp'oration of France Filed Oct. 24, 1965, Ser. No. 504,833 Claims priority, application France, Dec. 14, 1964, 998,922 5 Claims. (Cl. 260346.4)

ABSTRACT OF THE DISCLOSURE A process is described for the continuous liquid phase counter-current oxidation of aromatic hydrocarbons containing oxidizable side chains, especially the oxidizing of xylenes to various phthalic acids and/0r anhydrides. The process is carried out in a series of reactors in which the reaction fluids are maintained at approximately constant pressures and without condensation between reactors. Off gases are dehydrated by azeotropic distillation in such a manner that the dehydrating agent is recovered and unreacted aromatics and solvents are recovered in a dehydrated state suitable for recycling into the oxidation process. 7

The present invention relates to an improved process for liquid phase oxidation of aromatic hydrocarbons containing one or more aliphatic side chains.

There have been many proposed prior art processes directed to liquid phase oxidation of alkyl aromatic hydrocarbons with air or oxygen and it is well known that this type of oxidation process is favored by the presence of various halides and/or salts of heavy metals in the liquid phase. It is thus disclosed in U.S. Patent 2,276,774 the use of cobalt and manganese salts, alone or in the presence of heavy metal salts or halides, such as for example lead and barium bromide.

It is also known that ortho-xylene can be oxidized in the presence of an inert solvent. This ortho-xylene oxidation reaction is usually. conducted using temperatures ranging from 100 to 200 C., and using atmospheric or higher pressures in an aromatic hydrocarbon solvent, such as benzene or an aromatic carboxylic acid such as benzoic acid, or in an aliphatic acid solvent such as acetic acid, propionic acid, etc.

This reaction has already been conducted either in a discontinuous manneror in a continuous one in a single reactor, or, preferably, in several reactors put in series. This last kind of process is a countercurrent xylene oxidation process conducted in 2 or 3 reactors maintained at pressures and temperatures which increase in each successive reactor, with vapor condensation between every oxidation zone. and recycling of the mother-liquors from one reactor to another. These techniques call for a complex installation which requires numerous and delicate adjustments. Such techniques are disclosed in U.S. Patent 3,092,658.

A particular object of this invention is to provide a simpler, improved process for the oxidation of aromatic hydrocarbons with an oxidizing gas, and a more specific object of the invention isto provide a simpler, less expensive continuous process for the production of phthalic anhydride of great purity from ortho-xylene.

The new process of this invention uses relatively simple equipment, reactive conditions that are easier to control and provides a continuous process which produces very high yields of aromatic monoor polycarboxylic acids and/ or their corresponding anhydrides.

In its broadest concept the process of this invention comprises:

ice

l) Continuously oxidizing with an oxidizing gas, such as air, oxygen, etc., a liquid phase of an aromatic hydrocarbon having one or more oxidizable side chains in the presence of catalytic amounts of heavy metal halides and/or other heavy metal salts dispersed in an inert solvent such as an aliphatic carboxylic acid solvent, in a succession of reactors in which the reactive fluids flow countercurrently, partially or entirely, without condensation of the products between reactors, and without the addition of fresh solvent to the last reactor, while maintaining a practically uniform pressure in every reactor.

(2) In cases in which a hydrocarbon starting material is used which is able to form an anhydride or a mixture of anhydride and acid, the reaction mass should be hydrolyzed with water.

(3) Filtering and recycling the reaction product mother-liquors from the first reactor without previous dehydration.

(4) When the corresponding anhydride is desired, dehydrating the aromatic polycarboxylic acid obtained and purifying it.

The basic process and the variations outlined above are especially convenient for producing ortho-phthalic acid or anhydride from ortho-xylene and air, therefore xylenes are used in this disclosure to illustrate the general process of this invention. However, it should be clear that other aromatic acids or anhydrides could be prepared using the process of this invention.

The reaction medium used in accordance with the invention essentially contains an oxidizing gas such as air, an aromatic hydrocarbon such as xylene, an inert solvent such as an aliphatic carboxylic acid like acetic acid, and a catalyst.

The disclosure specifically illustrates the use of ortho-, metaor para-xylene and mixtures of ortho-, metaand para-xylene starting materials; however, it should be understood that other alkylaromatic hydrocarbons could be oxidized in a similar manner.

The respective proportions of alkylaromatic hydrocarbon (xylene) and acetic acid are preferably, but not necessarily, maintained between narrow limits, 5 to 30% by weight of hydrocarbon to 95 to 70% of acid being preferred without taking into account the amount of the catalytic system present in the reaction mixture.

The preferred catalysts used in this process are a mixture of hydrated heavy metal salts and/or halides, such as halides and/or other salts of cobalt and manganese. The most preferred catalyst mixture is one containing equal or different molar proportions of cobalt acetate, manganese chloride and barium bromide, but other heavy metal salts could be used effectively. The molar ratio of each of the three catalyst constituents is preferably between 1 and of moles per mole of ortho-xylene, which corresponds to 0.003 to 0.036 mole of catalyst per mole of hydrocarbon to be oxidized, but other ratios can be used.

The oxidation step of the process of this invention is carried out in a series of several reactors, for example, a series of three reactors may be used, the reaction liquid flowing from one reactor to another while the liquid is continually stirred with the help of recirculating pumps placed on the supply-circuits or with some other type of mechanical agitators. Each of the reactors should be built or lined with a corrosion resisting material suitable for the operational conditions incurred in actual use.

To obtain high acid yields using the process of this invention, the oxidation rates in every reactor should be maintained between optimum limits. Optimum oxidation rates will vary with the number and position in the series of the reactors employed. For example, when three reactors are used, it is best to maintain the oxidation 3 rate to less than 65% in the first reactor and less than in the third reactor.

The oxidation temperature may be between U0 and 230 C., and is preferably maintained between 140 and 200 C.

The pressure is maintained approximately constant in the oxidation system and the preferred pressures employed are between 3 and kg./cm. or more preferably between 4 and 10 kg./cm. in the temperature range indicated above.

Preferably the air required for oxidation is fed under suitable pressure through parallel conduits into the two last reactors so that the air or oxidizing gas runs countercurrently to the liquid aromatic hydrocarbon reaction mass. This division of the oxidation air flow in the two last reactors creates a thermic balance favorable to maximum recuperation of heat produced in the reaction. The excess hot gas which escapes from these last two reactors is sent as a whole to the first reactor through which it again passes countercurrently with respect to the liquid reaction mass. At the first reactor outlet, this hot gas is low in oxygen. The gas passes from the first reactor through an azeotropic column in which it is dehydrated with the help of a hydrocarbon, such as benzene or similar dehydrating material, and the unreacted xylene and acetic acid vapors are removed. The xylene and acetic acid recovered at the foot of the distillation column are recycled to the first oxidation reactor while the benzene/ water mixture obtained at the top of the column is sent to a condenser where it is removed as an aqueous phase separation.

Not having to condense vapors between each reactor reduces the possibility of the gas phase exploding and allows the direct use of the heat produced in the reaction to help run the azeotropic distillation column.

In a modification of this process, it is possible to cool all of the gas which escapes from the reactor and to run the azeotropic distillation with the totality of condensed liquid.

This process may be further modified by using a series of three reactors and distributing the total amount of air required for oxidation into each of the reactors; the hot gas coming out of the third reactor (in which the oxidation proportion is the lowest) is led into the first reactor (the one where the oxidation ratio is the highest) while the heat passing out of the second reactor is directly conducted into the azeotropic distillation column. The latter also received the hot gas from the first reactor. Such distribution of air, while producing a favorable thermic balance, offers the advantage of reducing notably the quantity of hot gas which gOes through the first reactor.

When the starting hydrocarbon is ortho-xylene, the reaction mass coming from the oxidation reactors contains essentially a mixture of ortho-phthalic acid and phthalic anhydride. The reaction mass is hydrolyzed while still at boiling point temperature and atmospheric pressure using known processes. The phthalic acid precipitate produced is removed by filtration, then, after a washing and centrifuging step, it is continuously dehydrated by heat at a temperature between 220 and 260 C. at atmospheric pressure. The recovered crude phthalic anhydride is then rectified continuously at reduced pressure, without previous chemical and/ or thermal treatment.

When the raw hydrocarbon is rnetaand/or paraxylene, the reaction mas will contain isoand/or terephthalic acid, which is separated by filtration, then washed and centrifuged.

The mother liquors from the phthalic acid filtration and the precipitate washing liquors are preferably recycled continuously to the first oxidation reactor, after adjustments of the concentrations to desired levels of orthoxylene, acetic acid and catalyst. By modifying this recycle procedure, it is possible to send a portion of the mother- 4 liquors to the other reactors to vary'in each the effective residence time of the reactive medium.

The chemical yields obtained in the oxidation zone are generally above 90%. In the case of ortho-xylene, the yields after dehydration and rectification of the acid obtained are above 97%.-

The invention will be further illustrated by referring to FIGURE I which shows a diagram of a preferred embodiment of the oxidation process of this invention.

The oxidation reaction takes place in each of the three reactors R R and R In the diagram shown in FIG- URE I, these reactors are at different levels with respect to one another, so that the reactive liquid may overflow from one reactor to another, the liquid passing through the overflow vessels 1 and 2 in the manner shown. The ortho-xylene, aliphatic carboxylic acid and catalyst are fed into reactor R through conduits 3 and 4, and the liquid flows from reactor R to reactor R to reactor R after passing through overfiow vessels 1 and 2, and conduits 5 and 6. The overflow tanks have branched connections with circulating pumps 7, 7 bis, and 8 which help regulate the flow in the manner illustrated.

The air necessary to complete the oxidation reaction is sent under pressure through pipe 9, through conduits 10 and 11, and the air enters the reactors R and R near the bottom. All of the excess air which escapes from the reactors R and R is sent to the bottom of reactor R through the conduit 12. Then the air passes through conduit 13 into distillation column D, into a condenser C, then into a decanter or settling tank C; from which it escapes through conduit 14.

Benzene is fed into the distillation column D through conduit 15 to wash and dehydrate the air that is now saturated with aliphatic acid, water and xylene vapors, which entered the column through conduit 13. The acid and xylene recovered at the bottom of the distillation column are fed directly through conduit 1.6 to reactor R The aqueous fraction removed in condenser C is drawn off through conduit 17 and the benzene fraction is recycled through conduit 18 into distillation column D.

The reaction mass withdrawn from conduit 19, containing a mixture of ortho-phthalic acid and phthalic anhydride, is subjected to successive operations of hydrolyzing, filtering of the acid phthalic precipitate, washing of this precipitate with acetic acid, centrifuging, dehydrating continuously, and rectifying the anhydride continuously under reduced pressure in known types of equipment not represented in the diagram.

The filtration mother-liquors and the acetic acid precipitate wash are mixed together, readjusted to the desired concentrations with added amounts of fresh reagent, then recycled through conduit 20' into the aromatic hydrocarbon feed conduit of primary reactor R In a further modification, the mother-liquors can be recycled, entirely or partially, through conduit 21 into column D.

According to the aforesaid modification of introducing air into each of the reactors (this modification is not shown in the drawing) the fresh air is conducted simultaneously to the bottom of the three reactors R R and R The hot gas coming from R is conducted into R while the gas from R and R is sent directly into the azeotropic distillation column D.

Specific examples which illustrate the process of this invention for producing carboxylic aromatic acids or their corresponding anhydrides as hereahove described are set forth below.

EXAMPLE I The ortho-xylene oxidation process was conducted continuously in a series of three reactors each of which were the same size, having avolume equal to 3 liters. The reaction mixture containing 20 parts by weight of 99.5% pure ortho-xylene and parts by weight of acetic acid as well as the catalyst was fed through conduit 3 into reactor R, at an average flow rate that varied between 1 and 1.5 l./hr.

The catalyst contained a mixture of hydrated barium bromide, hydrated manganese chloride and hydrated cobalt acetate in a quantity of moles of each constituent per mole of xylene to be oxidized.

The average air flow varied between 1000 and 1500 l./hr. at the inlet of the reactors R and R and 950 to 1200 l./hr. at the outlet which is conduit 14 emerging from the condenser C.

The pressure was approximately uniform throughout the oxidation apparatus at approximately 6 kg./cm. absolute pressure, and the temperature in the reactors was between 157 to 162 C. in R 152 to 157 C. in R and 147 to 152 C. in R After completing the oxidation reaction the reaction mass drawn off through conduit 19 was continuously hydrolyzed by boiling for 1 hour at atmospheric pressure in the presence of water, filtered, washed with acetic acid, centrifuged to remove phthalic acid precipitate, and dehydrated by heating for 2 hours at 235 to 240 C. at atmospheric pressure.

The crude phthalic anhydride obtained was then rectified in 2 columns, continuously at sub-atmospheric pressure.

The mother-liquors and the acetic acid from the phthalic acid precipitate wash were also recycled continuously through conduit 21 into column D, after readjustment to the concentrations set forth above by addition of fresh amounts of reagent, and orthoxylene is fed to reactor R through conduit 4.

The air containing acetic acid, xylene and water vapors leaves R through conduit 13 and enters column D where it was washed .with 900* to 1000 gr./hr. of benzene which was fed through conduit 15, and the recovered acetic acid and xylene was recycled continuously through conduit 16 to reactor R To complete the process, the aqueous phase taken from decanter C now free of acetic acid, was continuously drawn off at an average rate of 90* to 150 gr./h1'.

The table below indicates the conversion rates and the chemical yields obtained during oxidation at each recycling phase of the reactive mixture for a series of 7 mother-liquor recyclings:

The chemical yield is defined by the ratio:

Number of moles of obtained phthalic acid and anhydride Number of moles of introduced ortho-xylol X100 The phthalic anhydride produced was very pure. Its melting point was above 130.8 C., and its thermal stability, in degrees Hazen after heating at 250 C. for two and a half hours, was less than EMMPLE II This reaction was carried out using the same conditions, the same reagents and the same amounts of catalyst set forth in Example I, but meta-xylene was substituted for ortho-xylene.

After the oxidation was completed, the reaction mass drawn off through conduit 19 was continuously filtered; the isophthalic acid precipitate was washed with acetic acid and dried.

EXAMPLE III The procedures of Example II were repeated, but paraxylene was substituted for meta-xylene. After 8 recyclings, terephthalic acid having a purity greater than 98% and an average chemical yield of 97% was obtained.

EXAMPLE IV The continuous oxidizing procedures and conditions set forth in Example II were repeated with a mixture containing the following composition: m-xylene86.8% by weight; p-xylenel2%; and ethylbenzene1.2%. After 4 recyclings, a mixture of phthalic and benzoic acids, having an average yield of was obtained. This yield is defined by the ratio:

Weight of the obtained precipitate Weight of the theorical precipitate X100 calculated in isophthalic acid What is claimed is:

1. In a process for producing continuously benzene polycarboxylic acids and anhydrides by stage-countercurrently oxidizing polyalkylphenyl hydrocarbons with an oxygen-containing gas in the liquid phase, in the presence of a catalyst containing barium bromide jointly with cobalt and manganese salts and in the presence of aliphatic carboxylic acid solvent, then, in the case of anhydride production, by hydrolysing the reaction mixture, separating the obtained precipitate of aromatic polycarboxylic acids and by dehydrating said precipitate in a known manner, the improvement which comprises effecting said oxidation in a series of reactors under a substantially constant pressure between 3 to 15 kg./cm. at a temperature range of -230 C., preventing any condensation of vapors between the reactors and recycling continuously the recovered reaction product motherliquors into the first oxidation reactor and dehydrating azeotropically with the help of a hydrocarbon the gaseous stream coming out from the reactors.

2. Process of claim 1, wherein the oxidizing gas is sent in parallel into the two last reactors of the series, then the gaseous stream coming out from both of the said reactors is led entirely into the first reactor from which it is sent to an azeotropic distillation column.

3. Process of claim 1, wherein the oxidizing gas is simultaneously led into each of the reactors; the hot gas coming out from the last reactor is directed to .the first reactor, while the hot gas passing out from the first reactor and out from all the others except the last one is led directly into an azeotropic distillation column.

4. Process of claim 1, wherein the hydrocarbon used for azeotropic dehydration is benzene.

5. Process of claim 1, wherein the polyalkylphenyl hydrocarbon is selected from the group consisting of the dimethyl and trimethyl benzenes.

References Cited UNITED STATES PATENTS 3,092,658 6/1963 Baldwin et a1 260524 3,155,718 11/1964 Brown et al. 260-524 ALEX MAZEL, Primary Examiner.

B. I. DENTZ, Assistant Examiner. 

